Environmental campaigners said they had discovered plans for a large increase in the planting programme, which was much larger than expected.

Food safety minister Jeff Rooker: "The government said in April that for next year there would be an increase in GM tests"

Five large-scale trials are being carried out to study what effects GM crops have on wildlife, neighbouring crops and the environment. The plan is to extend these annually over four years.

But food safety minister Jeff Rooker said extra research needed to be carried out in realistic farming conditions.

He told BBC Radio 4's Today programme that the current test sites were "essentially to establish the methodology for monitoring in subsequent years.

"The government announced in April that for next year there would be 20 sites for the three crops of maize and the two types of oil seed rape," he said.

The government would work on 25 sites on the basis that there would be "20 good ones".

Environment Correspondent Margaret Gilmore: "The trials are part of a four-year experiment"

The anti-GM pressure group, Genewatch, said there are considerable "discrepancies" between the size and scope of what the government has admitted to and what is revealed in official documents.

Genewatch spokeswoman Dr Sue Mayer said a company developing a new form of oilseed rape resistant to herbicide intends increasing the area of farmland involved in trials.

'Wild crops could be harmed'

Opponents of GM crops fear the test crops could affect plants growing in the wild.

Dr Sue Mayer, of Genewatch: "There is a strange discrepancy about the figures"

The current trials are aimed at studying the impact of such crops on other species, and the Department of the Environment has said it is necessary for them to be conducted on a large scale.

No GM crops are grown commercially in the UK, although the government could decide to let companies do so from next year.

However, there is growing pressure on ministers to delay a decision until the end of the trial period.

But it is not just the testing of GM crops that has caused concern.

Shadow minister Andrew Lansley: "We have civil servants appearing to fight for a policy rather than provide information"

The Conservative party has written to the cabinet secretary, the most senior civil servant, to complain about the government setting up a special communications unit.

It says the unit will "manipulate the media" to get across a positive message on GM technology. The shadow minister who signed the letter is Andrew Lansley.

He told BBC Radio 4's Today programme that there was a distinction between ministers, who argue for their policy, and civil servants, "whose job it is to provide information".